The present invention relates to a plasma surface treatment system and a plasma surface treatment method, and particularly to a plasma surface treatment system and a plasma surface treatment method for introducing nitrogen into a surface of a substrate to be treated which is for use in production of a semiconductor device and which is provided with a silicon oxide film, a metallic oxide film or the like on the surface thereof.
MOS type silicon semiconductor devices have the problems as to scattering of the threshold voltage and as to suppression of the short channel effect, attendant on miniaturization of transistor structure. As a countermeasure against the problems, there have been developed surface channel type CMOS (Complimentary MOS) transistors having the so-called dual gate structure in which a gate electrode containing an N type impurity is used for the N channel MOS transistor and a gate electrode containing a P type impurity is used for the P channel MOS transistor.
Conventionally, the gate electrode on the PMOS side in a CMOS transistor having the dual gate structure has been produced by a method in which polycrystalline silicon is built up on a silicon oxide film to be a gate insulation film by a CVD process, and boron is introduced into the polycrystalline silicon by an ion implantation technique, followed by a heat treatment for activation.
Meanwhile, boron in the gate electrode on the PMOS side is instable against heat. Therefore, the heat applied during various thermal steps conducted after the formation of the gate electrode, such as a CVD process for forming a silicon nitride film and a source/drain activating anneal process, leads to the phenomenon in which boron in the gate electrode diffuses through the gate oxide film to the silicon substrate. This phenomenon is generally called “punch-through of boron”. Due to the punch-through of boron, depletion occurs on the gate electrode side, causing a decrease in the driving current of the transistor. In addition, boron having diffused to the substrate, or the channel region, causes such problems as scattering of the threshold value and worsening of sub-threshold characteristics. In view of this, generally from the 0.18 μm rule generation on, a technique of subjecting the silicon oxide film constituting the gate oxide film to a nitriding treatment to convert the oxide film into an oxynitride film, thereby suppressing the punch-through of boron, has been introduced.
Conventionally, it has been an ordinary practice to perform the nitriding treatment by a heat treatment in a high-temperature gas atmosphere of nitrogen oxide (NO), dinitrogen oxide (N2O), ammonia (NH3) or the like. Besides, attendant on miniaturization of design rules, there has been proposed a technique of introducing nitrogen shallowly into an extremely thin silicon oxide film by use of plasma composed mainly of nitrogen (N2) gas which is excited by a high-frequency electric field (see, for example, Japanese Patent Application No. 2002-1051 (page 4, FIG. 1)). The plasma nitriding treatment consists in generating a nitrogen-containing plasma to perform plasma nitriding of the silicon oxide film for 5 min under the conditions of a flow rate of nitrogen in the treating atmosphere of 200 cm3/min, a treating atmosphere pressure of 10 Pa, a substrate temperature of 600° C., and an RF power of 500 W.
It has been elucidated, through concentration measurement by secondary-ion mass spectrometry (SIMS) or the like, that nitrogen introduced into the silicon oxide film by a heat treatment or a plasma is normally present in the silicon oxynitride film formed and in the silicon substrate, with a nitrogen concentration peak at the interface between the silicon oxynitride film and the silicon substrate.
However, the nitrogen atoms introduced into the silicon substrate, or the channel forming regions, act as fixed electric charges and constitute a cause of scattering of the carriers, thereby deteriorating the mobility of the carriers. In addition, it has been pointed out that nitrogen present in the gate insulation film on the side of the interface with the silicon substrate has a great relation with the generation of NBTI (Negative Bias Temperature Instability) which has been becoming an important problem in recent years, particularly in miniaturized PMOS transistors. In order to solve this problem, it is necessary to lower, as much as possible, the concentration of nitrogen present in the channel regions and in the film near the interface with the silicon substrate. Namely, there is a request for a technology of implanting nitrogen into an extremely thin silicon oxide film in a desired concentration and to a desired depth, thereby forming a silicon oxynitride film with good controllability.
Meanwhile, it is easier to convert Si—Si and Si—H bonds into Si—N bonds than to convert Si—O bonds into Si—N bonds, on the basis of reactional energy. Upon a heat treatment in a high-temperature gas atmosphere of nitrogen oxide (NO), dinitrogen oxide (N2O), ammonia (NH3) or the like, nitrogen is concentrated in the channels and at the interface with the substrate. The reason is as follows. Since many Si—Si and Si—H bonds are present not only in the substrate but also in the so-called structure transition regions present in the silicon oxide film near the interface with the substrate, utilization of the heat treatment reaction causes a diffusion reaction, through which nitrogen reaches these regions to generate the Si—N bonds.
On the other hand, in the case of nitriding by a plasma using a high-frequency electric field which has been vigorously investigated in recent years, the diffusion reaction inevitable in the case of nitriding by use of the heat treatment reaction is suppressed greatly; therefore, the plasma nitriding shows a certain effect as means for lowering the nitrogen concentration in the channels and at the substrate interface. Unlike the diffusion phenomenon and the ion implantation technique, however, the plasma nitriding technique has the problem that the control method by an external input has not been fully elucidated and it is very difficult to control the output results. Therefore, in the case of nitriding an insulation film by use of plasma, rules of thumb by the workers in the art and much try-and-error activity are needed to obtain a desired nitrogen concentration and a desired nitrogen concentration gradient in the insulation film.